Rubber-modified styrenic resins (HIPS) are used widely in the fields of electric appliances, electronic equipment, office automation equipment, communication equipment and the like on account of their inherent qualities such as good impact resistance and easy processability.
In recent years, a growing demand for recycling of articles molded from resins has generated an increasing tendency to omit the painting of the surface of molded articles and created a strong demand for improvement in surface properties such as appearance and scratch resistance.
However, HIPS resins derive their impact resistance from rubbery polymers dispersed as particles in the styrenic resin phase and it is these rubber particles that cause unevenness on the surface of a molded article to lower the surface gloss and scratch resistance. As a result, HIPS resins find a limited use in applications requiring good appearance and scratch resistance.
In view of this, remedial measures such as reducing the diameter of rubber particles and controlling the particle size distribution have been proposed to improve the appearance of HIPS.
However, these measures were unable to satisfactorily balance the impact resistance and rigidity against the appearance and, most decisively, the fundamental problem here was that they were unable to provide HIPS with sufficient scratch resistance.
Addition of lubricants such as organopolysiloxanes for enhancing surface slipperiness has been proposed as a means to improve scratch resistance. However, this approach is not necessarily sufficiently effective for making improvement in scratch resistance and, besides, the additives are liable to deteriorate the appearance and stain the mold.
Moreover, a method which comprises compounding ABS resins with a scratch-resistant material such as methacrylic resins is known as a means to improve simultaneously the impact resistance, appearance and scratch resistance. However, this technique involving the use of ABS suffers from marked deterioration of the processability compared with HIPS.
Japan Kokai Tokkyo Koho Hei 6-25,507 (1994) discloses rubber-modified styrenic resin compositions (rubber-reinforced methacrylate-styrene copolymer compositions) with excellent surface properties comprising rubbery polymers dispersed as particles in a continuous phase of copolymers of styrenic monomer and (meth)acrylate monomer.
However, a sufficient improvement of the surface properties, particularly the scratch resistance, by the aforementioned rubber-reinforced methacrylate-styrene copolymer compositions requires copolymerization of a large quantity of (meth)acrylate monomer and this, in turn, presents another problem of deterioration of the processability. Keeping the processability at a given level necessitates control of the glass transition temperature of the matrix phase and this, in turn, necessitates additional copolymerization of butyl acrylate or the like. However, the additional copolymerization of butyl acrylate will lower the heat resistance of molded articles with the resultant narrowing of the range of their practical use and also will raise the cost.
Japan Kokai Tokkyo Koho Hei 6-157,863 (1994) proposes a method for utilizing the aforementioned rubber-reinforced methacrylate-styrene copolymers and HIPS as a mixture of the specified ratio.
The method, however, is not sufficiently effective for improving the scratch resistance and enhances the rigidity with difficulty on account of the rubber being dispersed as particles also in the methacrylate-styrene copolymers.
As described above, it has been difficult to prepare materials of good processability and surface properties such as appearance and scratch resistance and, moreover, materials of balanced impact resistance and rigidity starting from inexpensive styrenic resins.
On the other hand, a process for melt-blending two kinds or more of resins after completion of polymerization by means of an extruder is known for the preparation of rubber-modified styrenic resin compositions as disclosed in the examples of Japan Kokai Tokkyo Koho Sho 63-221,147 (1988) and Japan Kokai Tokkyo Koho Hei 6-157,863 (1994).
However, this process requires an increased number of manufacturing steps and is not necessarily desirable from the standpoint of production cost.
In particular, Japan Kokai Tokkyo Koho Hei 6-157,863 (1994) gives examples in which special procedures such as the following are described for the blending of rubber-reinforced styrenic resins and rubber-reinforced methacrylate-styrene copolymers: 1 blending of the reaction mixtures after completion of polymerization in the manufacturing step; 2 kneading of the molten resins emerging from the recovery step; and 3 adding molten resins of one kind through an extruder or the like to resins of another kind while in the manufacturing step. Any of these procedures, however, is concerned with the blending of different resins after completion of polymerization and it is difficult to control the structure of matrix resins. In view of the existing state of things, the present inventors have conducted intensive studies, found that the aforementioned problems can be solved by continuing the polymerization while allowing an aromatic vinyl monomer and a (meth)acrylate monomer to coexist in the polymerization system under a specified condition in the manufacturing step of rubber-modified aromatic vinyl resins, and completed this invention.
Accordingly, an object of this invention is to provide a process for preparing rubber-modified styrenic resin compositions easily and economically with good processability and surface properties such as appearance and scratch resistance and, furthermore, with a good balance between impact resistance and rigidity.